1. Field of the Invention
This invention relates to a reference voltage generator, and in particular to a stable reference voltage generator utilizing flash memory transistors which is particularly applicable for flash memory applications.
2. Description of Related Art
There are many applications for voltage reference generators which are stable regardless of changes in ambient temperature and supply voltage. Such devices have application in oscillator, timer and voltage regulation circuitry.
Flash memory arrays with precise on-chip voltage regulation require stable voltage references. Typically, voltage reference generators are fabricated as a part of the fabrication of the flash memory integrated circuit array. It is therefore desirable for such a reference voltage generator to be constructed using the same CMOS fabrication techniques utilized for making the remainder of the flash memory array.
One such reference voltage generator which uses flash memory cells in conjunction with a flash memory array is described in U.S. Pat. No. 5,339,292 entitled "Precision Voltage Reference."This patent describes a voltage reference circuit which includes a pair of flash memory cells, each having a different charge on the respective floating gates. Circuitry is provided for connecting each of the flash cells in parallel circuits in which equal currents are generated in an equilibrium condition. A circuit is provided for generating a voltage indicative of the current in each of the pair of parallel circuits. A differential amplifier responsive to the voltages in the parallel circuits, provides an output voltage to vary the current through the flash memory cells to bring the currents through the respective parallel paths into equilibrium, and to maintain constant reference voltage.